/*
 * Copyright (C) 2016-2017 Apple Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
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 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
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#ifndef WTF_AutomaticThread_h
#define WTF_AutomaticThread_h

#include "Box.h"
#include "Condition.h"
#include "Lock.h"
#include "Ref.h"
#include "ThreadSafeRefCounted.h"
#include "Threading.h"
#include "Vector.h"

namespace WTF {

// Often, we create threads that have this as their body:
//
//     for (;;) {
//         {
//             LockHolder locker(m_lock);
//             for (;;) {
//  [1]            stuff that could break, return, or fall through;
//                 m_condition.wait(m_lock);
//             }
//         }
//         
//  [2]    do work;
//     }
//
// When we do this, we don't always do a good job of managing this thread's lifetime, which may lead
// to this thread sitting around even when it is not needed.
//
// AutomaticThread is here to help you in these situations. It encapsulates a lock, a condition
// variable, and a thread. It will automatically shut the thread down after 1 second of inactivity.
// You use AutomaticThread by subclassing it, and put any state that is needed between [1] and [2]
// in the subclass.
//
// The terminology we use is:
//
// [1] PollResult AutomaticThread::poll()
// [2] WordResult AutomaticThread::work()
//
// Note that poll() and work() may not be called on the same thread every time, since this will shut
// down the thread as necessary. This is legal since m_condition.wait(m_lock) can drop the lock, and
// so there is no reason to keep the thread around.

    class AutomaticThread;

    class AutomaticThreadCondition : public ThreadSafeRefCounted<AutomaticThreadCondition> {
    public:
        static WTF_EXPORT_PRIVATE RefPtr<AutomaticThreadCondition> create();

        WTF_EXPORT_PRIVATE ~AutomaticThreadCondition();

        WTF_EXPORT_PRIVATE void notifyOne(const AbstractLocker &);

        WTF_EXPORT_PRIVATE void notifyAll(const AbstractLocker &);

        // You can reuse this condition for other things, just as you would any other condition.
        // However, since conflating conditions could lead to thundering herd, it's best to avoid it.
        // One known-good case for one-true-condition is when the communication involves just two
        // threads. In such cases, the thread doing the notifyAll() can wake up at most one thread -
        // its partner.
        WTF_EXPORT_PRIVATE void wait(Lock &);

    private:
        friend class AutomaticThread;

        WTF_EXPORT_PRIVATE AutomaticThreadCondition();

        void add(const AbstractLocker &, AutomaticThread *);

        void remove(const AbstractLocker &, AutomaticThread *);

        bool contains(const AbstractLocker &, AutomaticThread *);

        Condition m_condition;
        Vector<AutomaticThread *> m_threads;
    };

    class WTF_EXPORT_PRIVATE AutomaticThread : public ThreadSafeRefCounted<AutomaticThread> {
    public:
        // Note that if you drop all of your references to an AutomaticThread then as soon as there is a
        // second during which it doesn't get woken up, it will simply die on its own. This is a
        // permanent kind of death where the AutomaticThread object goes away, rather than the temporary
        // kind of death where AutomaticThread lives but its underlying thread dies. All you have to do
        // to prevent permanent death is keep a ref to AutomaticThread. At time of writing, every user of
        // AutomaticThread keeps a ref to it and does join() as part of the shutdown process, so only the
        // temporary kind of automatic death happens in practice. We keep the permanent death feature
        // because it leads to an easy-to-understand reference counting discipline (AutomaticThread holds
        // strong ref to AutomaticThreadCondition and the underlying thread holds a strong ref to
        // AutomaticThread).
        virtual ~AutomaticThread();

        // Sometimes it's possible to optimize for the case that there is no underlying thread.
        bool hasUnderlyingThread(const AbstractLocker &) const { return m_hasUnderlyingThread; }

        // This attempts to quickly stop the thread. This will succeed if the thread happens to not be
        // running. Returns true if the thread has been stopped. A good idiom for stopping your automatic
        // thread is to first try this, and if that doesn't work, to tell the thread using your own
        // mechanism (set some flag and then notify the condition).
        bool tryStop(const AbstractLocker &);

        bool isWaiting(const AbstractLocker &);

        bool notify(const AbstractLocker &);

        void join();

    protected:
        // This logically creates the thread, but in reality the thread won't be created until someone
        // calls AutomaticThreadCondition::notifyOne() or notifyAll().
        AutomaticThread(const AbstractLocker &, Box<Lock>, RefPtr<AutomaticThreadCondition>);

        // To understand PollResult and WorkResult, imagine that poll() and work() are being called like
        // so:
        //
        // void AutomaticThread::runThread()
        // {
        //     for (;;) {
        //         {
        //             LockHolder locker(m_lock);
        //             for (;;) {
        //                 PollResult result = poll();
        //                 if (result == PollResult::Work)
        //                     break;
        //                 if (result == PollResult::Stop)
        //                     return;
        //                 RELEASE_ASSERT(result == PollResult::Wait);
        //                 m_condition.wait(m_lock);
        //             }
        //         }
        //
        //         WorkResult result = work();
        //         if (result == WorkResult::Stop)
        //             return;
        //         RELEASE_ASSERT(result == WorkResult::Continue);
        //     }
        // }

        enum class PollResult {
            Work, Stop, Wait
        };

        virtual PollResult poll(const AbstractLocker &) = 0;

        enum class WorkResult {
            Continue, Stop
        };

        virtual WorkResult work() = 0;

        // It's sometimes useful to allocate resources while the thread is running, and to destroy them
        // when the thread dies. These methods let you do this. You can override these methods, and you
        // can be sure that the default ones don't do anything (so you don't need a super call).
        virtual void threadDidStart();

        virtual void threadIsStopping(const AbstractLocker &);

    private:
        friend class AutomaticThreadCondition;

        void start(const AbstractLocker &);

        Box<Lock> m_lock;
        RefPtr<AutomaticThreadCondition> m_condition;
        bool m_isRunning{true};
        bool m_isWaiting{false};
        bool m_hasUnderlyingThread{false};
        Condition m_waitCondition;
        Condition m_isRunningCondition;
    };

} // namespace WTF

using WTF::AutomaticThread;
using WTF::AutomaticThreadCondition;

#endif // WTF_AutomaticThread_h

